JP-A-2004-258281 describes a technique of adjusting a developing image by utilizing a density correction image. Specifically, density correction images of two or more types are used to perform density measurements for from low density to high density on a belt (conveyance member), and a developing voltage is adjusted according to the measurement results of the density.
As described in JP-A-2004-258281, the density correction images of different densities are used to carry out the appropriate density adjustment for an intermediate color. However, it cannot be said that the adjustment of the developing image can be sufficiently carried out for image data having a low pixel, density (the number of pixels per a predetermined area), such as thin line and isolated pixel. That is, when the density correction images of different densities are simply used, it is not possible to carry out the sufficient adjustment on the developing image of the image data for forming an image having a low pixel density.